1. Field of the Invention
The present invention relates to a process for producing hexafluoropropylene oxide (hereinafter sometimes referred to simply as HFPO).
2. Discussion of Background
HFPO is a compound useful as a starting material monomer for fluorinated resins or rubbers, and it is produced industrially by an oxidation reaction of hexafluoropropylene (hereinafter referred to simply as HFP). HFPO contains, as impurities, unreacted HFP, water, hydrogen fluoride (hereinafter referred to simply as HF), hexafluoroacetone (hereinafter referred to simply as HFA), perfluoro acid fluorides such as perfluoropropionyl fluoride (hereinafter referred to simply as PFPF) and perfluoroacetyl fluoride, carbonyl fluoride, etc. These impurities include not only ones formed in the process for producing HFPO but also ones formed during the storage in a metal container.
Among the above-mentioned impurities, HFA and perfluoro acid fluorides act as chain transfer agents and thus hinder the polymerization reaction during the polymerization of HFPO. Further, water and HF also act as substances to hinder the polymerization reaction. Therefore, in a case where HFPO is used for a polymerization reaction, it is necessary to remove water, HF, HFA and perfluoro acid fluorides, contained as impurities, before its use.
As a method for removing water in HFPO, a method of passing HFPO through molecular sieves in a gas phase, is known (U.S. Pat. No. 3,412,148). Further, as a method for removing water, HF, HFA and perfluoro acid fluorides in HFPO, a method is known (JP-A-57-175185) wherein a metal hydroxide is packed at a preliminary stage and a metal hydride is packed in a subsequent stage, whereby HFPO is passed therethrough in a gas phase. On the other hand, a method is also known (Japanese Patent 3,306,957) wherein in order to prevent conversion from HFPO to HFA during the production or storage of HFPO, water or a water-containing compound is permitted to be present.
However, the above methods have various drawbacks. For example, in the method disclosed in U.S. Pat. No. 3,412,148, isomerization of HFPO to HFA is likely to take place by a catalytic activity of molecular sieves. Further, molecular sieves are likely to be fluorinated by HF, whereby not only the isomerization will be accelerated, but there will be a problem that decomposition of HFPO will proceed.
On the other hand, in the method disclosed in JP-A-57-175185, water will be produced as a by-product by the metal hydroxide at the preliminary stage, and in order to remove such water, it is required to combine the metal hydride at the subsequent stage, whereby not only the apparatus is required to be large sized to increase the installation costs, but also a labor is required to change packing materials for both the preliminary and subsequent stages. Further, the metal hydride is expensive, which makes the running cost high, and in addition, there is a drawback that hydrogen is produced as a by-product.
Further, in the method disclosed in Japanese Patent 3,306,957, HFPO and water are reacted to form a hydrate of pyruvic acid and HF, as disclosed by Nobuo Ishikawa in Organic Synthetic Chemistry Vol. 35, No. 2, p. 133 (1977). Formed HF will isomerizes HFPO to HFA as disclosed in JP-B-1-61090, and consequently there is a drawback that HFA will be increased.